1. Field of the Invention
The present invention relates to a synchronization protecting system for signals received in a radio base station.
2. Description of the Related Art
With the spread of recent movable or cellar communication systems, there is a demand for more reliability on a radio channel quality. To meet the demand, it is required to protect a radio channel quality by a synchronization protecting method having high ability.
In a prior method for protecting synchronization of signals received in a radio base station, which has been developed by the present applicant, a first synchronized word detecting window AP1 for the use to establish an initial synchronization is provided to cover a position of a synchronized word SW of a signal received from a cellar station, which is prescribed for a reference timing for transmission in the radio base station.
The synchronized word SW is detected in the first synchronized word detection window AP1. On the other hand, the position of synchronized word SW of the reception signal transmitted from the cellar station differs in every time a signal is received, according to a distance between the radio base station and the cellar station or a state of propagating radio waves. Accordingly, a width of the first synchronized word detecting window AP1 for use to establish an initial synchronization requires a width enough to cover the difference of the positions of the synchronized words SW of reception signals to certain extent.
However, once the synchronization is established, it can be predicted that the position of detecting the synchronized word SW is not widely changed at the time when the synchronized word SW is detected, namely, when the synchronization establishment is maintained after that.
Alternatively, to prevent from detecting the synchronized word by mistake in the first synchronized word detecting window AP1 having a wider width, a second synchronized word detecting window AP2, of which width is narrower than that of the first synchronized word detecting window AP1 is provided, based on a position of detecting an initial synchronized word detected in the first synchronized word detecting window AP1.
In other words, the second synchronized word detecting window AP2 protects the synchronized word SW detection, which is detected in the first synchronized word detecting window AP1. Once the second synchronized word detecting window AP2 becomes effective at the time of establishing an initial synchronization, a relationship of position with the first synchronized word detecting window AP1 is not changed, until there is no need to maintain the synchronization by finishing the communication with the cellar station.
In the method developed by the present applicant before, there is no means to change the position of setting the second synchronized word detecting window AP2 at the time when the position of detecting the synchronized word is larger than the width of the second synchronized word detecting window AP2, because the timing of receiving the signal from the cellar station is widely changed in the state the synchronization is once established.
Although the first synchronized word-detecting window AP1 can detect the synchronized word SW, the position of detecting the synchronized word is not within the second synchronized word-detecting window AP2. Therefore, that brings a problem such that the result of detecting in the second synchronized word-detecting window AP2 becomes inactive state.